The present invention relates to a novel process using an improved liquid collector assembly for phase separation of a two-phase (aqueous and organic) reaction product. The reactor and collector assembly are particularly applicable to the separation of caustic and hydrocarbon phases comprising the reactor effluent liquid streams in processes for the sweetening of sour petroleum fractions.
The sweetening of sour petroleum distillates involves improving their odor quality through the oxidation of mercaptan compounds (thiols) contained therein to disulfides. Of significant industrial importance in this field is the fixed-bed catalytic oxidation process described in U.S. Pat. No. 2,988,500. In this treatment process, mercaptans contained in petroleum distillates (e.g. sour gasoline) are reacted with an oxidizing agent (e.g. air) in the presence of an alkaline reagent (e.g. caustic solution). The oxidation reaction occurs upon passing this two-phase (hydrocarbon/aqueous) liquid mixture containing the dissolved oxidizing agent over a fixed bed of catalyst. An issue of primary importance therefore relates to the efficiency of separation of the liquid phases after completion of the sweetening reaction.
In conventional distillate treating technology, this separation occurs primarily external to the reactor, where a settler vessel allows the reactor effluent to establish phase equilibrium. The heavier aqueous phase containing the alkaline reagent, once a sufficient level is established in the settler, can be intermittently recycled to the reactor inlet. In this case the recycled caustic solution is recombined with the petroleum distillate feed and oxidizing agent before the reactor inlet. Typically, the same charge of caustic solution can be recycled several times for use in the oxidation reaction, before disposal and replacement with fresh material are required.
Newer developments in treating, however, have led in many cases to a more economically favorable operation using continuous, once-through flow of the alkaline reagent. This has been achieved through the minimization of caustic solution usage and consequently the reduction of caustic flow through the reactor. A major process modification associated with this xe2x80x9cminimum alkalinityxe2x80x9d mode has been a change in the point of separation between the aqueous and organic phases. Whereas the conventional technology relied on a separation external to the reactor, the modified reduced caustic flow processes allowed for the direct collection of separate phases after completion of the mercaptan oxidation reaction. Generally, these later-developed processes have been most suitable for the sweetening of relatively light hydrocarbon fractions such as gasoline.
The removal of reaction products as essentially pure aqueous and hydrocarbon streams after the sweetening reaction has been accomplished using an appropriately designed liquid collector assembly. The primary function of the liquid collector has been to maximize the surface area of conduits through which the hydrocarbon phase of the reaction mixture is forced to pass before exiting the reactor. During collection, the distribution of the treated hydrocarbon over a broad area has been found to promote essentially complete disengagement of the caustic solution from the hydrocarbon phase. This desired effect has been attributed to the reduction of the liquid flux (flow rate per unit area) through conduits of the liquid collector assembly. Spent caustic solution, which constitutes the heavier phase of the reaction mixture, normally flows by gravity past the collector assembly. This stream is withdrawn through a caustic drain located at the bottom of the reactor, below the liquid collector assembly where the hydrocarbon portion is separated and removed. In the interest of product quality, it is generally required that the treated, or sweetened, hydrocarbon product contain less than 1 ppm by weight of the metal cation (e.g. sodium) used for the caustic solution.
A second design consideration for the liquid collector assembly has been the separation of the treated hydrocarbon from the catalyst particles used in fixed-bed mercaptan oxidation processes. This is achieved using conduits of the liquid collector assembly that have perforated surfaces, where perforations of the appropriate size are fabricated according to methods known in the art.
A common type of liquid collector assembly comprises a plurality of cylindrical conduits extending horizontally into the reactor catalyst bed about a common transverse plane of the reactor at a constant height above the caustic drain. The cylindrical conduits have perforated surfaces, are each closed at one end, and are each in common flow communication with a piping manifold at the opposite end. The perforated conduit surfaces therefore promote both the hydrocarbon/caustic and liquid/solid phase separations required for the hydrocarbon sweetening process. Typically, flanged connections are used between the piping manifold and conduits, so that damage to the perforated surface of any particular conduit is easily remedied through replacement without welding. The main drawback of using laterally extending conduits, however, has been the limited allowable surface area for effecting the phase separation of hydrocarbon and aqueous components.
Another type of collector that is less frequently used is a so-called xe2x80x9cbasketxe2x80x9d construction where a cylindrical surface having a diameter smaller than that of the reactor and concentric with the reactor is affixed to the lower section of the reactor wall. The basket extends along a portion of the reactor length, allowing the separation of the treated hydrocarbon from the caustic solution and catalyst particles to take place in the annular region between the perforated conduit surface and reactor wall. A well-recognized problem with such basket-type designs is their substantial maintenance requirement. Any damage to the perforated surface of the conduit necessitates careful welding procedures to restore the surface properties. Removal of the basket for repairs is impractical when it is a permanently affixed structure within the reactor. Furthermore, inspection of the reactor wall section that is covered by the basket becomes impossible. This unavailability of a portion of the reactor wall is recognized as a significant drawback, since examination of the reactor inner surface is very important for purposes such as the detection of corrosion.
The reactor of the present invention applies to processes where organic and aqueous liquids are combined to carry out a reaction and thereafter must be phase separated as completely as possible. Applicant has determined that the use of a liquid collector assembly with conduits extending vertically along part of the vessel length allows for a relatively large surface area over which to cause the necessary phase separation. This collector design contributes to enhanced separation efficiency through the orientation of the conduits parallel to the liquid flow, which prevents the direct impingement of liquid flowing normal to the perforated surface. Also, the positioning of the piping manifold directly above the conduits further shields the conduits to some extent from the direct liquid flow path. Additionally benefiting the desired phase separation is the collection of reaction liquid from within a catalyst bed at radially and axially dispersed points. Lastly, unlike the previously mentioned basket design, maintenance and replacement of conduits are simple procedures.
Aside from design simplicity and ease of maintenance, however, the main advantage of the novel reactor and associated liquid collector design of the present invention lies in the increased liquid throughput allowable for a given reactor size. Until now, the reactor liquid linear velocity, which is calculated by dividing the total liquid volumetric flow rate by the reactor cross sectional area, has been limited, due to the decreasing caustic separation efficiency accompanying increasing flux through the liquid collector assembly conduits. With the use of vertical conduits, surface area is increased, so that the flux for a given flow rate is reduced. In fixed-bed mercaptan oxidation processes in particular, the present invention can now permit operation with superficial liquid velocities approaching values at which the caustic solution and hydrocarbon feed will begin to emulsify. This occurs typically at about 15-30 meters per hour, depending on a number of process variables. Alternatively, the reactor length to diameter ratio (L/D) for a given capacity hydrocarbon sweetening operation can be increased using the reactor of the present invention, resulting in greater design flexibility. This allows for the use of conventional reactor sizes and consequently significant cost savings, since it is usually cheapest to design a reactor of a given volume with an L/D of about 2-4.
In one embodiment, therefore, the present invention is an improved reactor for treating a sour hydrocarbon containing a mercaptan. The reactor comprises a vertically oriented vessel and a particle retention space for retaining a fixed bed of catalyst particles. The vessel also comprises an inlet that communicates with the upper portion of the vessel for contacting a downwardly flowing liquid with the fixed bed. A drain communicates with the bottom of the vessel for withdrawing a first portion of a liquid from the bed. A piping manifold extends into the particle retention space and communicates with a plurality of dependent conduits having perforated surfaces and extending vertically along part of the vessel length. The vessel further comprises a manifold outlet that communicates with the manifold, where the manifold outlet is for withdrawing a second portion of a liquid from contact with the fixed bed.
In a preferred embodiment the present invention is an improved reactor as described above, where the piping manifold, horizontally traversing the particle retention space, comprises a plurality of pipes extending radially from a common connection providing flow communication among the pipes. A plurality of dependent conduits extend downwardly from each pipe for communication therewith and the conduits have perforated surfaces comprising profile wire, fritted metal, or slotted metal.
In another embodiment the present invention is a process for treating a sour hydrocarbon containing a mercaptan. The process comprises contacting the hydrocarbon, oxidizing agent, and alkaline reagent at oxidizing conditions with a fixed bed of catalyst effective to oxidize the mercaptan and yield a treated hydrocarbon product and a spent reagent. A plurality of perforated surfaces, extending vertically over a portion of the fixed catalyst bed and dispersed over the catalyst bed cross section, is used to collect the treated hydrocarbon product. A piping manifold is then used to withdraw this product.